Piezoelectric actuators have long been recognized as highly desirable for use in systems requiring extremely fast mechanical operation in response to an electrical control signal. For this reason piezoelectric actuators are now receiving considerable attention by designers of fuel supply systems for internal combustion engines. Such designers are continually searching for ways to obtain faster, more precise, reliable and predicable control over the timing and quantity of successive fuel injections into the combustion chambers of internal combustion engines to help meet the economically and governmentally mandated demands for increasing fuel economy and reduced air pollution. If such goals are to be attained, fuel control valves must be designed to provide extremely fast and reliable response times.
When used as a valve actuator, piezoelectric devices are known to provide extremely fast, reliable characteristics when calibrated to and operated at a relatively constant temperature. However, internal combustion engines are required to operate reliably over an extremely broad ambient temperature range. Moreover, fuel injection valves mounted directly on the engine are subjected to an even broader range of temperatures since the operating temperatures of an internal combustion engine may extend well above ambient temperatures and may reach as much as 140.degree. C. or more. Such temperature extremes can produce wide variations in the operating characteristics (e.g. length of stroke and/or reaction time) of a piezoelectric actuator. Such actuator variations can lead to wide variations in timing and quantity when the piezoelectric actuator is used to control fuel injection into an internal combustion engine.
Numerous attempts have been made to overcome the problem of thermally induced variations in piezoelectric actuator operation. For example, U.S. Pat. No. 4,284,263 to Newcomb (assigned at issuance to U.S. Philips Corporation) discloses a control valve including a piezoelectric actuator which is temperature compensated by the provision of material having a high coefficient of thermal expansion in series with the piezoelectric material to match the coefficient of thermal expansion of the surrounding actuator housing. While this approach reduces temperature induced variation in operating characteristics, the length of the actuator assembly is by necessity substantially increased as compared with actuators which are not temperature compensated in this manner.
Another approach is disclosed in U.S. Pat. Nos. 5,740,969 and 5,819,710 (assigned at issuance to Mercedes-Benz) wherein two different materials having coefficients of thermal expansion both above and below that of piezoelectric material are placed in series to form the actuator housing having an effective coefficient of thermal expansion approximating that of the piezoelectric material. While the size of this assembly is not necessarily greater than an uncompensated piezoelectric actuator, the complexity and strength of the actuator may be compromised. For example, the housing illustrated in the '696 patent includes an upper housing portion 5a formed of Invar having a low coefficient of thermal expansion and a lower housing portion 5b formed of steel having a high coefficient of thermal expansion. This arrangement requires the housing portions to be joined end to end and thus adds to the cost of the resulting assembly and provides a possible point of weakness or failure. Other piezoelectric actuators have been disclosed with similar types of temperature compensation such as illustrated in U.S. Pat. No. 5,205,147 to Wada et al.
A still more complicated approach is disclosed in U.S. Pat. No. 5,875,764 to Kapel et al. (assigned at issuance to Siemens Aktiengesellschaft) including a hydraulic system for compensating for thermal growth in a piezoelectric actuator for a valve. While suitable for the purposes disclosed, this reference fails to disclose a simple mechanical system for automatically compensating for temperature changes and fails to suggest a method for easily overcoming the defects of the prior art.
Other techniques for temperature compensating piezoelectric devices are disclosed in the following references
U.S. Pat. No. 5,571,363 PA1 U.S. Pat. No. 5,376,860 PA1 U.S. Pat. No. 4,825,117 PA1 Japanese Patent No. 61-258485 PA1 Japanese Patent No. 62-36884
None of the references noted above discloses a technique for providing a simplified method for temperature compensating a piezoelectric actuator in a manner to overcome the short comings of the prior art.